Death march of a segmented and trilobate bilaterian — elucidates early animal evolution

From time to time, these GEITP pages have discussed animal (and plant) evolution — which was of course heavily influenced by gene-environment interactions. Given the climate and conditions on various regions of Earth some 2 billion years ago, living cells were “pushed” into multicellular animals (and plants). Molecular clock estimates predict that bilaterian animals [i.e. most animals, including the majority of phyla (but not sponges, ctenophores, placozoans, and cnidarians); bilateral embryos have three germ layers — endoderm, mesoderm, and ectoderm — and they have front and rear ends, and dorsal and ventral, and left and right sides] diverged in the Cryogenian Period (geological period dominated by glaciers and occurring prior to the Cambrian, when the fossil record shows a so-called ‘explosion’ of most major groups of complex animals, ) or the Ediacaran Period (spans 94 million years from the end of the Cryogenian Period 635 million years ago (MYA), to the beginning of the Cambrian Period 541 MYA), and many bilaterian clades (groups of organisms believed to have evolved from a common ancestor) had diverged before the close of the Ediacaran period; if this statement is correct, these estimates mandate the existence of motile bilaterians in the Ediacaran period. In part, this prediction is met by trace fossils from the late Ediacaran period — which include many ichnofossils (i.e. tracks, trails and burrows).

However, the phylogenetic importance of these ichnofossils is limited by the dearth of morphological details regarding the animals that produced them. To date, only a few Ediacaran taxa — Dickinsonia, Kimberella, and Yorgia — have been demonstrated to be capable of movement and producing ichnofossils. For these genera (subgroups), evidence for motility comes from mortichnia (fossil records of the animal’s final trail) that are produced shortly before death; these fossils are preserved together, with the producers of the traces. However, these Ediacaran organisms mostly produced discrete and isolated footprints; so far, only Kimberella is known to be capable of producing continuous trails.

Thus, the identity of these animals — that produce most of the long and continuous trails that date to the Ediacaran period — remains unknown. This limited knowledge about motile Ediacaran bilaterians greatly undermines efforts of evolutionary scientists to grasp the biological basis for a transformative evolutionary innovation that ultimately led to the Cambrian, and the agronomic (plants for food, fuel, fiber, and land restoration) “revolutions”, which has had lasting effects on the evolution of all segmented life since (including us humans).

Authors [see attached article] describe Yilingia spiciformis, an animal fossil from an area in south China, where the rocks there were dated to 551–539 MYA. Y. spiciformis is preserved with its locomotion traces, including a mortichnium (‘death march’). The traces show an elongated bilaterian animal having evidence of body segmentation, segment polarity, and directional locomotion. Yilingia offers critical insights into the identity of Ediacaran trace-makers — as well as the early evolution of body segmentation and bilaterian motility (i.e. left legs and right legs). Yilingia is possibly related to the panarthropod (group of segmented ecdysozoans arranged into three phyla: Arthropoda, Onychophora, and Tardigrada) or annelids (worms), and sheds light on the origin of segmentation in bilaterians. As one of the few Ediacaran animals demonstrated to have produced long and continuous trails, Yilingia provides intriguing insight into the identity of animals responsible for Ediacaran ichnofossils. 😊



· Nature Sept 2019; 573: 412-415

This entry was posted in Center for Environmental Genetics. Bookmark the permalink.